The present invention relates generally to circuit breakers and more particularly, an electrically conductive dual connector strap for connection of either an electronic trip unit or a mechanical trip unit (e.g. thermal magnetic or magnetic).
Circuit breakers are one of a variety of overcurrent protective devices used for circuit breaker protection and isolation. The basic function of a circuit breaker is to provide electrical system protection whenever an electrical abnormality occurs in any part of the system. In a rotary contact circuit breaker, current enters the system from a power line. The current passes through a load strap to a stationary contact fixed on the strap and then to a moveable contact. The moveable contact is fixedly attached to an arm, and the arm is mounted to a rotor that in turn is rotatably mounted in a cassette. As long as the fixed contact is in physical contact with the moveable contact, the current passes from the fixed contact to the moveable contact and out of the circuit breaker to down line electrical devices.
In the event of an extremely high overcurrent condition (e.g. a short circuit), electromagnetic forces are generated between the fixed and moveable contacts. These electromagnetic forces repel the movable contact away from the fixed contact. Because the moveable contact is fixedly attached to a rotating arm, the arm pivots and physically separates the fixed contact from the moveable contact, thus tripping the unit.
Protection against persistent and instantaneous overcurrent conditions is provided in many circuit breakers by a thermal-magnetic trip unit having a thermal trip portion, which trips the circuit breaker on persistent overcurrent conditions, and a magnetic trip portion, which trips the circuit breaker on short-circuit conditions.
In order to trip the circuit breaker, the thermal-magnetic trip unit activates an operating mechanism. Once activated, the operating mechanism separates the fixed and moveable contacts to stop the flow of current in the protected circuit. Conventional trip units act directly upon the operating mechanism to activate the operating mechanism. In a mechanical thermal-magnetic trip unit, a bimetal element is connected with the associated electric circuit for persistent overcurrent detection. If a long-term overcurrent condition causes the bimetal to reach a predetermined temperature, the bimetal bends and unlatches the operating mechanism to trip the circuit breaker. A magnetic trip unit is employed for instantaneous overcurrent detection. In other words, the magnetic element interrupts the circuit when a high level of overcurrent persists for a short, predetermined period of time. Modern magnetic trip units include a magnet yoke (anvil) disposed about a current carrying strap, an armature (lever) pivotally disposed proximate the anvil, and a spring arranged to bias the armature away from the magnet yoke. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet yoke. The magnetic field acts to rapidly draw the armature towards the magnet yoke, against the bias of the spring. As the armature moves towards the yoke, the end of the armature contacts a trip lever, which is mechanically linked to the circuit breaker operating mechanism. Movement of the trip lever trips the operating mechanism, causing the fixed and moveable contacts to open and stop the flow of electrical current to a protected circuit.
Some circuit breakers employ an electronic trip unit to provide persistent and/or instantaneous overcurrent detection. Electronic trip units are well known. Electronic trip units typically are comprised of current sensors that provide analog signals indicative of the power line signals. The analog signals are converted by an A/D (analog/digital) converter to digital signals which are processed by a microcontroller. The trip unit further includes RAM (random access memory), ROM (read only memory) and EEPROM (electronic erasable programmable read only memory) all of which interface with the microcontroller. The ROM includes trip unit application code, e.g., main functionality firmware, including initializing parameters, and boot code. The EEPROM includes operational parameters for the application code. When the signal received by the electronic trip unit indicates an overcurrent condition, an output of the electronic trip unit actuates an electromechanical actuator, which in turn, unlatches the operating mechanism to trip the circuit breaker. Conventional circuit breaker devices with electronic trip units utilize a current transformer disposed around one of the current carrying straps within the circuit breaker. The current transformer performs two functions. First, it provides operating power to the trip unit circuitry.
For a given model of circuit breaker, various types of trip units may be used. For example, mounted within a circuit breaker housing, a mechanical trip unit (e.g. thermal-magnetic or magnetic) can be employed. Alternatively, an electronic trip unit can also be employed that utilizes a current transformer. In order to accommodate the various trip units that can be selected within an electrical distribution system, different types of mechanical connections to conductors (straps) are required based on the type of trip unit employed. Further, in order to simplify manufacturing, it is desired to have the ability for late point identification of the type of trip unit to be employed.